The electrical basis of excitation and inhibition in the rat anoccygeus muscle.

1. The transmembrane potential of smooth muscle cells of the rat anococcygeus muscle was studied with micro-electrodes. The muscle had a mean resting membrane potential of --61-5 mV and normally lacked spontaneous mechanical and electrical activity. 2. Stimulation of intramural nerves with single pulses produced a small depolarization and contraction. At frequencies greater than 0.5 Hz a second component occurred which had a maximum value of 35--51 mV with short trains at 30 Hz. Spike potentials were rarely seen. Depolarization was accompanied by a decrease in membrane resistance. 3. Noradrenaline (3 times 10-minus 5 M) and guanethidine (3 times 10-minus 5 M) both depolarized the membrane and produced contraction. Initially, oscillations in the membrane potential were oftern seen. 4. Intramural nerve stimulation in the presence of guanethidine damped the oscillations and produced relaxation. This was accompanied by neither hyperpolarization nor a change in membrane resistance. 5. TEA (1 mM) depolarized the membrane (--47-6 mV) and initiated spontaneous activity. Field stimulation evoked spikes with overshoot.